[O] New beamer exporter: top-aligned columns

[James Harkins]

According to beameruserguide.pdf, I can give an option "t" to a
columns environment to align the columns by the top line of text. It's
not clear to me how to put this into org.

So far, I have:

*** Columns                                                       :B_columns:
    :PROPERTIES:
    :BEAMER_env: columns
    :END:

Where should the "t" go? Some :options string, I suppose.

Thanks.

hjh